1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus configured to fix a toner image on a recording medium by passing the recording medium, on which an image has been transferred, between a fixing member and a pressurizing member.
2. Description of the Related Art
In recent years, with the advancement in development of image forming apparatuses such as printers, copiers, and fax machines, there has been increasing demand for power saving and high speed. To meet this demand, it is important to improve the thermal efficiency of the fixing device used in the image forming apparatus.
In an image forming apparatus, an unfixed toner image is formed by an image forming process such as electrophotographic recording, electrostatic recording, and magnetic recording. The unfixed toner image is formed on a recording medium such as a recording sheet, a printing sheet, a photosensitive sheet, and an electrostatic recording sheet, by an image transfer method or a direct method. As the fixing device for fixing the unfixed toner image, a contact heating type fixing device is widely used, which uses a heat roller method, a film heating method, an electromagnetic induction heating method, etc.
A fixing device using the heat roller method has a basic configuration including a rotational roller pair formed by a fixing roller and a pressurizing roller that is held together via pressure contact with the fixing roller. In the fixing roller, a heat source such as a halogen lamp is provided, so that the fixing roller is adjusted to have a predetermined temperature. The recording material is guided into and conveyed by the contact part, i.e., a fixing nip part, between the two rollers of the rotational roller pair. Accordingly, the unfixed toner image is fused and then fixed by the heat and pressure from the fixing roller and the pressurizing roller (see, for example, patent documents 1 and 2).
In a fixing device using the film heating method, the recording material is brought into close contact with a heating body fixed to a supporting member, via a thin, heat resistant fixing film. The heat of the heating body is supplied to the recording material via the fixing film while sliding the fixing film against the heating body. In the fixing device, a ceramic heater is used as the heating body. The ceramic heater includes a resistance layer placed on a ceramic substrate made of a material such as alumina and aluminum nitride, having heat resistant properties, insulation properties, highly thermally-conductive properties, etc. This fixing device can use a thin fixing film having a low thermal capacity. Therefore, this fixing device has higher heat transfer efficiency than the fixing device using the heat roller method. Accordingly, with this fixing device, the warm-up time is reduced, so that quick start and power saving can be achieved.
In a fixing device using the electromagnetic induction heating method, an eddy current is induced to a metal layer (heat generating layer) of a fixing sleeve by magnetic flux, and Joule heat is generated by the eddy current.
In the fixing device using the electromagnetic induction heating method, a direct fixing film is caused to generate heat by using an inductive current. With this fixing device, it is possible to perform a fixing process with higher efficiency than that of a fixing device using the heat roller method using a halogen lamp as the heat source.
A well-known configuration of a fixing device using the electromagnetic induction heating method includes a fixing sleeve having a release layer, an elastic layer, and a metal layer (heat generating layer). Inside the fixing sleeve, a fixing roller is provided, which is formed of an elastic layer and a supporting member (core). The fixing roller and a pressurizing roller are brought into pressure contact with each other via the fixing sleeve to form a pressure contact nip part.
In this configuration, the fixing sleeve is prevented from moving in the thrust direction by being adhered to the fixing roller with a silicone adhesive. When the fixing sleeve is not adhered to the fixing roller, a ring having a larger diameter than the fixing sleeve is provided at the end of the fixing roller to prevent the fixing sleeve from moving.
In the device described in patent document 2, the toner image is fixed onto the recording material at the fixing nip part formed by the fixing roller and the pressurizing roller. This recording material may be wound around the fixing roller or the pressurizing roller due to the viscosity of the toner that has been fused but not yet cooled, or due to the direction of the fixing nip. Thus, in order to properly guide the recording material to the correct conveying path, a separating member has been used in the conventional technology.
In the past, a contact type claw has been used as a separating member. However, in full-color images, claw marks created on the roller appear in the images. Therefore, in recent years, a non-contact type separating plate is widely used. In the non-contact method, it is important to manage the separation gap. Therefore, a metal sheet is typically used as the non-contact separating plate to achieve the required positioning precision. Furthermore, a sheet-type separating plate is used instead of a claw, to prevent the image quality from deteriorating due to rubbing the recording material.
Furthermore, to precisely form the separation gap between the recording material and the roller, a pushing part is provided at an end part outside the range of the image. The pushing part may be combined with the separating plate or formed separately from the separating plate. By pushing the pushing part against the roller, the separation gap is formed.
As described above, in recent fixing devices, in order to achieve power saving, it is important to rapidly heat the fixing device to quickly reach a standby status. For this purpose, even in fixing devices included in image forming apparatuses used for mass production, rollers of small diameters are used to reduce the heat capacity. In this case, with respect to the linear speed, the separating plate/pushing part slides against the same part of the roller many times.
Furthermore, as the roller is rapidly heated, the thermal expansion of the roller may not occur in a uniform manner in the longitudinal direction, particularly immediately after activation or after sheets of small sizes have continuously passed through the rollers. Generally, the center part of the roller in the longitudinal direction becomes larger than the end parts, and the roller tends to become shaped like a drum. Thus, it may become difficult to push the pushing part of the separating plate against the outer periphery of the roller by a uniform force in the longitudinal direction. Accordingly, the pushing part may only push the end parts of the drum-shaped roller in the width direction.
Furthermore, in the case of the electromagnetic induction heating method, a fixing sleeve having a metal layer is used, and therefore the pushing part pushes against the outer periphery of the roller that is harder than a conventional fixing roller. Due to the above reasons, in fixing devices, the surface of the fixing roller tends to be damaged by the separating plate/pushing part.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-79219
Patent Document 2: Japanese Laid-Open Patent Publication No. 2009-128575